The present invention relates to an acrylic polymer used for acrylic rubber, a pressure-sensitive adhesive, an alkali-soluble adhesive, an alkali-soluble film, an alkali-soluble injection molding, water ink, a coating, a low profile agent, a dispersing agent, a fiber-processing agent, a sealing agent, a vibration-controlling material, a resin improver, an asphalt additive and so forth, and also relates to use of the polymer and a process for producing it.
An acrylic polymer is used for acrylic rubber, a pressure-sensitive adhesive, an alkali-soluble adhesive, an alkali-soluble film, an alkali-soluble injection molding, water ink, a coating, a low profile agent, a dispersing agent, a fiber-processing agent, a sealing agent, a vibration-controlling material, a resin improver, all asphalt additive and so forth.
Also, because an acrylic polymer of high acid value is soluble in an alkali, it is used in a dissolved condition for ink, floor polish, coating of paper, paint, an adhesive, a pressure-sensitive adhesive. Also, because the polymer of high acid value has the advantage that it can be recovered by dissolve it in an alkali, it is used for a molding material, a vessel, film, a label and so forth.
An acrylic polymer is generally produced by carrying out a radical polymerization reaction of a monomer component containing an acrylic acid-based monomer as a main component till reaching a high polymerization conversion. The radical polymerization is generally carried out by the emulsion polymerization, suspension polymerization, solution polymerization, or bull polymerization.
The emulsion polymerization, suspension polymerization, and solution polymerization have an advantage that, because the polymerization is carried out by dispersing a monomer into a dispersion medium, polymerization temperature can be easily regulated and a reaction solution is easy to run even when the polymerization conversion has become high. However, they have following defects.
In the emulsion polymerization and suspension polymerization, a precipitating process (which is necessary in the case of emulsion polymerization), a filtering process and a drying process are necessary for isolating a polymer from a dispersion medium, the procedure is complex and productivity is poor, and in addition, a surfactant such as an emulsifier or a dispersing agent mingles into an obtained acrylic polymer and a pure polymer is difficult to obtain. An acrylic polymer in which the surfactant remains is inferior in water resistance and, according to the use, also inferior in strength or cohesion. If a macromolecular acrylic polymer is produced by the emulsion polymerization or suspension polymerization, a gel portion is formed and fluidity is poor. In carrying out a copolymerization reaction by the emulsion polymerization or suspension polymerization, according to the desired composition of a polymer, to selectively employ plural kinds of monomers of different hydrophilic properties is necessary, however, because there is a case where emulsifying or suspending is impossible, the obtainable polymer has composition limitation.
In the solution polymerization, because a large amount of an organic solvent is used as a dispersion medium, there is a trend that a large amount of low molecular weight products are formed and that the molecular weight distribution (which is also called "polydispersity coefficient" or "polydispersity index") of acrylic polymers becomes broad, and heat resistance or processing operation performance of the polymers lowers, so that, according to the use, disadvantage occurs. Also, if a large amount of an organic solvent is used, because the solvent must be volatilized in order to isolate the polymer, productivity is poor.
In contrast, because the bulk polymerization does not use any dispersion medium and does not need the aforementioned surfactants, it has not such defects as mentioned above and is an industrially advantageous process. However, if a polymerizable monomer of which polymerization reactivity is very intensive is subjected to the bulk polymerization, this polymerization is hard to control. Therefore, according to the sort of monomers, conversion into a polymer of high molecular weight and designing the molecular weight distribution may become difficult, products gelated (gelled) or deteriorated may be formed due to rapid and intensive exothermic polymerization, or, in the worst case, explosion occurs.
Among polymerizable monomers, styrene and methyl methacrylate have been known in that their bulk polymerization can be controlled, and this polymerization reaction has been studied from old times and industrialized.
A styrene-based polymer is industrially obtained by carrying out a bulk polymerization reaction of styrene or a monomer mixture mainly containing it up to high polymerization conversion in the presence or absence of a polymerization initiator and then by volatilizing a small amount of residual unreacted styrene. Also, although styrene can be mildly polymerized by a thermic polymerization reaction in the absence of a polymerization initiator, a mercaptan is occasionally used for molecular weight control. In this case, a chain transfer reaction of the mercaptan is active, so that a polymer of broad molecular weight distribution is formed.
In the bulk polymerization reaction of methyl methacrylate, methyl methacrylate or a monomer mixture mainly containing it is polymerized in the presence of a polymerization initiator. However, because it is difficult to obtain a polymer having a narrow molecular weight distribution in the case of high polymerization conversion, the polymerization is stopped at the stage of such a low polymerization conversion as about 60% and a large amount of unreacted monomer is volatilized. This is because the polymerization rate of methyl methacrylate is faster than that of styrene and, in addition, if the polymerization conversion becomes high, the polymerization rate is accelerated owing to a gel effect (which is a phenomenon that the polymerization rate becomes fast when the viscosity of a reaction mixture becomes high with progress of a polymerization reaction) and uneven temperature distribution occurs in a reaction medium, and therefore, a part, where a polymerization reaction does not so proceed, and a part, where a polymerization reaction rapidly and intensively proceeds, coexists.
As mentioned above, even in a case of methyl methacrylate of which polymerization reaction is far milder compared with that of an acrylic acid-based monomer such as methacrylic acid, acrylic acid and an acrylic acid ester, the bulk polymerization reaction is stopped at a stage of low polymerization conversion. Therefore, a bulk polymerization reaction of the acrylic acid-based monomer of which polymerization reaction is more intensive than that of methyl methacrylate is difficult to control reaction temperature, and generally, it is not practically used at all. In Japanese Official Patent Gazette Nos. showa 62-41523 and heisei 2-55448, there has been proposed the bulk polymerization reaction of an acrylic acid-based monomer using a screw extruder. The bulk polymerization of an acrylic acid-based monomer proposed in these gazettes comprises: introducing a polymerization initiator and the acrylic acid-based monomer into the screw extruder of specific structure, and then raising viscosity in a short time by taking advantage of rapid and intensive reaction so as to transfer a solution with a screw. In Japanese Official Patent Gazette No. showa 55-401, there has been proposed a bulk polymerization reaction comprising using an organic mercaptan in the presence of oxygen.
In a case of using an acrylic polymer of high molecular weight for various uses, a high molecular weight component in the polymer displays necessary performance and a low molecular weight component i n the polymer becomes an origin of performance-lowering. On the other hand, in a case of using an acrylic polymer of low molecular weight for various uses, a low molecular weight component in the polymer displays necessary performance and a high molecular weight component in the polymer becomes an origin of performance-lowering. Accordingly, an acrylic polymer having a narrow molecular weight distribution is desired.
According to the methods in the above-mentioned Japanese Official Patent Gazette Nos. showa 62-41523 and heisei 2-55448, because temperature of the reaction solution does not rapidly rise but stepwise rises as the polymerization conversion becomes higher, the bulk polymerization reaction of an acrylic acid-based monomer can be carried out without causing a runaway reaction. However, an obtained acrylic polymer contains a large amount of low molecular weight component and its molecular weight distribution Mw/Mn is very large such as about 8 to 13, so that heat resistance or processing operation performance is poor. For example, in a case of using the polymer as a polymer for coating, a low molecular weight component lowers coating film strength and contamination resistance or, if a high molecular weight component is contained more than necessity, the viscosity of solution is high, so that operation performance is bad. In addition, because the bulk polymerization in said Japanese Official Patent Gazette Nos. showa 62-41523 and heisei 2-55448 uses a rapid and intensive reaction of an acrylic acid-based monomer, its practically use is possible on a small scale such as a laboratory level, but difficult on an industrial scale from viewpoint of safety.
On the other hand, according to the method in said Japanese Official Patent Gazette No. showa 55-401, heat generation can be relatively easily controlled, however, oxygen exists, so that the gas phase part is in a range of explosion and there is a danger of explosion which may occur due to the presence of an ignition source. Even if this danger could be avoided, the polymer is colored very much. Therefore, the method is not preferable one as a method for producing an acrylic polymer which is characterized by transparency.
Currently, it is actual circumstances that there is no skill of producing industrially with good productivity and safely an acrylic polymer which has a narrow molecular weight distribution and does not contain the above-mentioned surfactant.